Ch12−p1
Chapter 12 Alkanes & Cycloalkanes
Alkanes
We begin our study of organic chemistry with the
simplest class of compounds – the hydrocarbons. As the
name implies, hydrocarbons are compounds that contain
only carbon and hydrogen atoms. The first class of
hydrocarbons that we will study are the alkanes, which
are characterized by carbon-carbon single bonds. The
alkanes are also called saturated hydrocarbons because
they cannot add any more hydrogen atoms to the
structure.
The general formula for alkanes is CnH2n+2.
e.g.
C atoms
C1
C2
C3
C10
H atoms
H2(1)+2 = 4
H2(2)+2 = 6
H2(3)+2 = 8
H2(10)+2 = 22
Alkane
CH4
C2H6
C3H8
C10H22
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Nomenclature.
IUPAC – International Union of Pure and Applied
Chemistry.
The IUPAC determines the protocol for naming organic
compounds. For naming continuous-chain alkanes, a
prefix is used to describe the number of carbon atoms
followed by the suffix –ane.
e.g.
Alkane
CH4
C2H6
C10H22
Prefix
methethdec-
Suffix
-ane
-ane
-ane
Name
methane
ethane
decane
Look at Table 12.1. You should know these prefixes and
names.
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1.
2.
3.
4.
5.
6.
Summary of IUPAC Rules for Nomenclature
Find the most important functional group (MIFG)
in the molecule according to the priority ranking HO
list. Note the suffix.
I
“-one” × 2 = “-dione”
Find the longest carbon chain (or ring) that has all
of the functional groups.
Number this carbon chain from the end closest to
1
the first MIFG. (Number a ring starting at the
2
MIFG.) If a ‘tiebreaker’ is necessary, number in HO 3
4
the direction that will give the next most
I 5
important functional group the lowest number. If,
6
7
after working through all functional groups, there
is still a tie, use the ‘alphabetical order’ tiebreaker.
If the MIFG could be in more than one position,
include the number of the carbon it is attached to.
If there is more than one MIFG, include numbers
for both positions.
“-2,6-dione”
If they are not the MIFG, look for double and/or
triple bonds. To number a double or triple bond, HO
use the smaller number from the two carbon
4
I
atoms involved.
Name the main chain (or ring), attaching the
MIFG suffix to the end of the name. If this gives
a name in which the next letter after ‘e’ is another
vowel, drop the ‘e’.
“-4-heptene-2,6-dione”
O
I
O
O
I
O
O
I
O
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7. Find, name and number the remaining groups
attached to the main chain. If a functional group HO
appears more than once, use a prefix to indicate
how many there are and include a number for each I
appearance.
“3-hydroxy” & “4,5-diiodo”
8. List the functional groups in alphabetical order
(not counting prefixes) followed by the main
chain (or ring) name.
“3-hydroxy-4,5-diiodo-4-heptene-2,6-dione”
9. Using the priority rules, assign R- or S- to any chiral A
HO
carbons that have the stereochemistry shown. If
there is more than one chiral carbon, use a number I
to indicate the carbon to which each R or S
corresponds.
“S-3-hydroxy-4,5-diiodo-4-heptene-2,6-dione”
10. Using the priority rules, assign cis- or trans- if it HO
applies. This is not necessary for cycloalkanes if
I
you have assigned R- or S- to the carbon atoms
involved.
“trans-S-3-hydroxy-4,5-diiodo-4-heptene-2,6-dione”
O
I
O
O
C
B
I
O
O
I
O
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Functional Group Priority Ranking and Suffixes
Aldehyde (-al)
Ketone (-one)
Alcohol (-ol)
Thiol (-thiol)
Ether (no special suffix)
Alkene or Alkyne (-ene or -yne)
Haloalkane (no special suffix)
Alkane or Cycloalkane (-ane)
***remember special names for benzene rings***
Functional Group Prefixes
methyl
-OCH3
methoxy
-CH3
-CH2CH3
ethyl
-OCH2CH3
ethoxy
-CH2CH2CH3
propyl
-OCH2CH2CH3
propoxy
-CH(CH3)2
isopropyl
-OCH(CH3)2
isopropoxy
-CH2CH2CH2CH3
butyl
-OCH2CH2CH2CH3
butoxy
-CH(CH3)CH2CH3 sec-butyl -OCH(CH3)CH2CH3 sec-butoxy
-CH2CH(CH3)2
isobutyl
-OCH2CH(CH3)2
isobutoxy
-C(CH3)3
tert-butyl
-OC(CH3)3
tert-butoxy
-F
fluoro
phenyl
-Cl
chloro
-Br
bromo
H
-I
iodo
C
benzyl
-OH
hydroxy
=O
oxo
2
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Alkyl Groups in Branched Hydrocarbons.
So far we have seen how to name linear-chain alkanes but
how do we name branched-chain alkanes? In the IUPAC
system, hydrocarbon substituents are named as alkyl
groups. The alkyl group is named by replacing the –ane
ending of the corresponding alkane name with –yl.
Notice how the alkyl group has one less hydrogen atom
than the alkane it is derived from. Consider the molecule
below; it has two alkyl groups (positions 2 and 4)
branched from the main chain of six carbon atoms.
CH3
CH3
H2C
1
6
4
2
3
5
The alkyl group at position 2 (CH3) is derived from
methane (CH4) and so is named methyl. Likewise, the
group at position 4 (CH3CH2) is derived from ethane
(CH3CH3) and is named ethyl. Be sure to look at and
know Table 12.5.
Naming Branched-Chain Alkanes.
When using the IUPAC system for naming hydrocarbons,
the longest continuous chain or main chain contains the
compound root word. The substituents are then numbered
according to their position on the main chain.
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Step 1.
Since we are only looking at alkanes, which are at the
bottom of the priority list, we go straight to finding the
longest continuous chain and name it as the main chain
(Rule 2 and 6).
CH3
CH3
CH
CH2
CH2
CH3
pentane
Step 2.
Identify the substituents on the main chain by their
smallest position number. Use a prefix (di-, tri-, tetra-) to
indicate a group that appears more than once. When
multiple substituents allow numbering from both ends of
the main chain, use the direction that gives the lowest
series of numbers. In the name, hyphens separate
numbers from words and commas separate numbers (Rule
7).
CH3
CH3
CH
CH2
CH2
CH3
1
2
3
4
5
CH3
CH3
CH2
CH
CH
CH3
4
3
2
1
H3C
5
CH3
H3C
CH
4
2,3-dimethylpentane
(not 3,4-dimethylpentane)
CH3
CH2
C
CH3
CH3
5
2-methylpentane
(not 4-methylpentane)
3
2
1
2,2,4-trimethylpentane
(not 2,4,4-trimethylpentane)
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Step 3.
When different substituents are present, list them in
alphabetical order. The prefixes for repeated substituents
are not used in deciding alphabetical order (Rule 8).
H3C
CH3
3-ethyl-2-methylpentane
(not 2-methyl-3-ethylpentane)
(not 3-ethyl-4-methylpentane)
CH3
CH2
CH
CH
CH3
5
4
3
2
1
CH2
CH3
CH
CH
CH2
CH2
CH3
3
4
5
6
7
H3C
CH3
CH2
CH
3-ethyl-2,4-dimethylheptane
(not 2,4-dimethyl-3-ethylheptane)
CH3
1
2
Give IUPAC names for the following molecules
H3C
CH3
CH2
CH
CH
CH
CH3
CH2
CH3
CH
CH3
CH3
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Conformation of Alkanes.
An important property of carbon-carbon single bonds is
the rotation about this bond. Rotation about a carboncarbon single bond gives rise to different arrangements of
molecules called conformers.
It is important to understand that conformers are the same
molecule in different arrangements. Consider butane.
Recall from Chapter 11 that compounds with the same
molecular formula but with a different arrangement of
atoms are called constitutional isomers. For example, for
the formula C4H10 there are two possible arrangements:
CH3
CH3
CH2
CH2
CH3
CH3
CH
CH3
Remember; since each isomer has a different arrangement
of atoms they have different properties.
e.g. Draw all the constitutional isomers for hexane and
give their IUPAC names.
Ch12−p10
Classifying Carbon Atoms in Hydrocarbons.
The carbon atoms in hydrocarbons can be classified
according to the number of carbon atoms bonded to it. A
primary carbon (1°) is a carbon atom bonded to only one
other carbon atom. A secondary carbon (2°) is bonded to
two other carbon atoms. A tertiary carbon (3°) is bonded
to three other carbons, and a quaternary carbon (4°) has
four bonds to other carbons.
1° carbon
1° carbons
4° carbon
CH3
CH3
C
CH3
CH2
CH3
CH
CH3
2° carbon
3° carbon
1° carbons
Classify the carbons in the following molecules as
primary, secondary, tertiary, or quaternary.
CH3
CH3
CH
CH3
Ch12−p11
Drawing Structures from IUPAC Names.
If you are given the IUPAC name of a compound you
have all the information necessary to draw its structure.
Start by identifying the compound root name, and then
add the substituents at the indicated positions. Consider
2,3-dimethylhexane. The name can be dissected into its
individual components.
substituent positions
2,3
di
number of similar
substituents
substituent
methyl
functional group identifier
hex
ane
compound root name
Draw the condensed structure for 4-ethyl-2,2dimethyloctane.
Cycloalkanes.
Up to now we have only considered straight-chained or
branched-chained saturated hydrocarbons. Of course,
alkanes can also form cyclic structures called
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cycloalkanes. Cycloalkanes, with general formula CnH2n,
have two fewer hydrogen atoms than the corresponding
alkanes. Some examples are shown in Table 12.6.
Naming cycloalkanes is similar to straight-chain alkanes
except that the prefix cyclo- is added to the name of the
alkane. When one substituent is present, the substituent
name is placed before the cycloalkane name. No number
is needed for one substituent. When two or more
substituents are present, the numbering starts by assigning
carbon 1 to the substituent that gives the lowest series of
numbers.
CH3
H2C
8
1
2
7
1-ethyl-4-methylcyclooctane
not 1-methyl-4-ethylcyclooctane
nor 1-ethyl-6-methylcyclooctane
3
6
4
5
CH3
CH3
methylcyclobutane
2
H3C
CH3
1
3
4
6
5
1,3-dimethylcyclohexane
not 1,5-dimethylcyclohexane
Ch12−p13
Since the cycloalkanes are in a cyclic arrangement,
rotation about the carbon-carbon single bonds within the
ring is impossible. This gives the cycloalkanes two
distinct sides or ‘faces’. As a consequence, this
interesting characteristic gives rise to cis-trans isomers.
Cis-trans isomers only differ in the orientation of atoms in
space. Consider the molecule 1,2-dichlorocyclopropane.
The structure can be written with the chlorine atoms on
the same side (cis isomer) or with the chlorine atoms on
opposite sides (trans isomer). Look at the molecules
carefully. Convince yourself that the two isomers
represent different molecules.
Cl
H
Cl
H
cis-1,2-dichlorocyclopropane
Cl
H
H
Cl
trans-1,2-dichlorocyclopropane
Physical Properties of Alkanes and Cycloalkanes.
Alkanes and cycloalkanes are nonpolar (why?) and
therefore insoluble in polar solvents such as water. They
typically have densities around 0.6 to 0.7 g/mL which is
lower than that of water 1.0 g/mL. This is why oil will
float on top of water.
Since all the atoms in alkanes are carbon and hydrogen
(which have very close electronegativity values), small
Ch12−p14
dipoles are generated throughout the molecule which
result in very weak intermolecular forces. As the number
of carbon atoms increase, the amount of intermolecular
forces also increases. Although these forces are relatively
weak, the cumulative effect of these intermolecular forces
does lead to a gradual progression through the different
states of matter.
C1-C4 carbons → gases
C5-C17 carbons → liquids
C18 and greater → solids (greases, waxes, asphalt)
The boiling points for branched-chained alkanes tend to
be lower than straight-chained alkanes with the same
number of carbons. This is because branched-chain
alkanes are more compact which reduces the amount of
intermolecular forces (due to less surface area).
Cycloalkanes on the other hand, have slightly higher
boiling points. Because rotation of the carbon-carbon
bonds is restricted, cycloalkanes are more rigid than the
straight- or branched-chained alkanes. This rigidity
allows the cycloalkanes to ‘stack’ together and therefore
increase the amount of intermolecular forces between
molecules.
The alkanes are invaluable to our everyday life. As
shown above, the first four alkanes (methane, ethane,
propane, and butane) are gasses and are used extensively
as heating fuels. Alkanes having 5-8 carbon atoms are
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liquids and are used in fuels such as gasoline. Alkanes
containing 9-17 carbon atoms are also liquids but have
higher boiling points. This makes them useful in
kerosene, diesel, and jet fuels. Alkanes with 18 or more
carbon atoms are waxy solids at room temperature. These
hydrocarbons, known as paraffins, are used to make
waxes, petroleum jelly (Vaseline) and other ointments.
The different alkanes are typically obtained from crude
oil. Crude oil is found as a mixture of different
hydrocarbons which is refined by fractional distillation.
As shown in Table 12.8, many useful products are
obtained from crude oil.
Chemical Properties of Alkanes and Cycloalkanes.
Because alkanes are made up of carbon-carbon single
bonds, which are difficult to break, they are the least
reactive family of organic compounds. However, this
does not mean that they are inert. As evident from their
extensive use as fuels, saturated hydrocarbons react
(burn) readily in oxygen. They also undergo reactions
with the halogens (typically with Cl and Br).
You have already seen the combustion reaction in Chapter
6. Alkanes undergo combustion when they react with O2
to form CO2, H2O, and heat. The general reaction is
shown below:
Ch12−p16
alkane
CO2
+ O2
+ H2O +
heat
The combustion of alkanes is not always complete
however. If the supply of oxygen is insufficient, carbon
monoxide (CO) will form instead of carbon dioxide
(CO2). Carbon monoxide is a colourless, odourless,
poisonous gas and can be lethal if produced in a non
ventilated area. CO is poisonous because it binds to
hemoglobin stronger than oxygen. Hemoglobin is
responsible for transporting oxygen to our cells and
organs. Therefore, if the concentration of CO is high
enough in a person, that person will suffocate.
The incomplete combustion of methane is shown below:
2CH4 + 3O2
2CO + 4H2O + heat
Haloalkanes.
A subclass of the alkanes is the haloalkanes. Haloalkanes
are alkanes in which one or more hydrogen atoms are
replaced by halogen atoms. The halogens are named as
substituents:
Halogen
Name
fluorine
chlorine
bromine
iodine
fluorochlorobromoiodo-
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Simple haloalkanes still use their common names, some
of which do not indicate their structure. They are named
as alkyl halides. Some examples are shown below.
IUPAC
CH3
CH3
Cl
CH2
Br
Common
chloromethane
methyl chloride
bromoethane
ethyl bromide
F
CH3
CH
CH3
2-fluoropropane
isopropyl fluoride
Cl
Cl
C
H
C
Cl
H
H
dichloromethane
methylene chloride
Cl
Cl
trichloromethane
chloroform
Cl
C
Cl
Cl
Cl
tetrachloromethane
carbon tetrachloride
Ch12−p18
Halothane, CF3CHClBr, is a widely used general
anesthetic. What is its IUPAC name?
Synthesis and Reactions of Haloalkanes.
Beside combustion reactions, alkanes can also undergo
free radical halogenation reactions. The reaction of an
alkane with a halogen is called a substitution reaction
because the reaction involves the replacement of one or
more hydrogen atoms with halogen atoms (compare with
replacement reaction, section 6.4). These reactions are
done in the presence of ultraviolet light.
H
H
hν
H
C
H
+
Cl
Cl
C
H
H
+
Cl
HCl
H
In the presence of excess light and chlorine, the reaction
will continue until all the hydrogen atoms are replaced.
H
H
C
H
Cl
Cl
Cl
Cl2
H
C
Cl
H
+ HCl
Cl2
H
Cl
C
Cl
Cl
+ HCl
Cl2
Cl
C
Cl
Cl
+ HCl
Ch12−p19
With longer chain alkanes, any of the hydrogen atoms can
be substituted giving rise to many isomers. However,
because 3° hydrogens are more reactive than 2°
hydrogens which are more reactive than 1° hydrogens, the
product distribution can be unequal.
2° carbon
CH3
CH2
Br
Br2
CH3
CH3
CH
CH3
+
CH3
CH2
CH2
Br
+ HBr
40%
60%
1° carbon
Most of you are familiar with haloalkanes.
Chlorofluorocarbons (CFCs) are haloalkanes which are
used as propellants and refrigerants. Two common CFCs,
Freon 11 and Freon 12 are shown below.
Cl
Cl
C
Cl
F
Cl
Freon 11
C
F
F
Cl
Freon 12
CFCs are non toxic, non flammable, unreactive, and
extremely robust. However, in the mid 70s, F. S.
Rowland and M. J. Molina showed that CFCs were
responsible for the depletion of the ozone layer. They
showed that in the stratosphere, CFCs broke down to give
chlorine radicals (Cl·) which started a chain reaction.
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These radicals reacted with ozone molecules (O3) to give
oxygen molecules. More importantly, however, the
chlorine radicals were regenerated and started the whole
process again. It has been estimated that one chlorine
radical can destroy as many as 100,000 ozone molecules.
The reactions are shown below using Freon 12 as an
example.
Chain initiation
Cl
Cl
Step 1
F
Cl
C
hν
F
+
C
Cl
(chlorine radical)
F
F
Chain propagation
Step 2
Step3
Cl
ClO
+
O3
(ozone)
+ O3
+
ClO
Cl
+
O2
2O2
Ch12−p21
Important Concepts from Chapter 12
• IUPAC System for Nomenclature
Prefixes (di, tri, tetra…)
Suffixes (-ane)
Branched alkanes (methyl, ethyl…)
• Conformation of Alkanes
• Carbon Atom Classification
• Reactivity of Alkanes
• Haloalkanes
• Reactivity of Haloalkanes
1°, 2°, 3°, and 4°.